Hook support for a closed circuit fluid cooled gas turbine nozzle stage segment

ABSTRACT

A nozzle stage segment includes inner and outer bands spaced radially one from the other with a nozzle vane therebetween. The outer band includes a wall defining a portion of the hot gas path flow through the turbine and an outer cover defining a chamber with the outer wall. The vane includes a vane extension integrally cast with the nozzle segment and extending into a vane extension on the cover. The cover mounts a forward hook for structurally mounting the nozzle segment to a fixed part of the turbine casing, thereby establishing a structural load path through the hook, the cover extension and the vane extension upon welding the extension to one another and the outer wall and cover to one another. Cavities in the vane open into the chamber to receive a cooling medium and which cavities and chamber form part of a closed circuit cooling path through the nozzle stage segment.

BACKGROUND OF THE INVENTION

The present invention relates to supports for gas turbine nozzle stages having closed circuit cooling, for example, steam cooling, and particularly relates to a hook for supporting a closed circuit, steam cooled nozzle stage segment from a fixed portion of the turbine casing.

Closed circuit steam cooled nozzle stages for a gas turbine typically have an annular array of nozzle vane segments each having inner and outer bands with one or more nozzle vanes extending generally radially between the bands. To provide a closed circuit cooling system, each of the bands has a chamber for containing the cooling medium, e.g., steam for cooling the walls of the nozzle stage. The vane between the chambers is divided into cavities and the cooling steam flows from the outer chamber through the cavities for cooling the vane and into the chamber of the inner band for cooling the inner wall. The spent cooling steam then flows through the inner band chamber generally radially outwardly through one or more cavities of the vane to a cooling steam exhaust.

More particularly, as illustrated in U.S. Pat. No. 5,634,766, of common assignee herewith and for each nozzle segment, the outer band comprises an outer wall and a radially outward cover defining the outer chamber between the wall and cover. The cooling steam is supplied through an inlet in the cover and through an impingement plate in the chamber for impingement cooling of the outer wall. The cooling steam then flows through apertures in a cast extension of the vane extending through the outer chamber. From the apertures, steam is directed into inserts in one or more flow cavities in the vane for transmitting the steam through apertures in the inserts for impingement cooling the vane walls, particularly the leading edge. The inner band comprises the inner wall and a radially inner cover and receives the spent cooling steam from the vane. The spent cooling steam reverses direction and flows through apertures in an impingement plate in the inner chamber for impingement cooling of the inner wall. The spent cooling steam flows radially outwardly through an insert in another cavity in the vane for impingement cooling and then through the vane extension of the outer band to a steam exhaust outlet.

From the foregoing, it will be appreciated that the closed loop cooling circuit requires a cover and a wall for each of the outer and inner bands to contain the cooling steam. The nozzle stage segments are also hung from the outer fixed casing of the turbine by forward and aft hooks typically formed integrally with the outer wall of the nozzle stage segment. Particularly, the forward hook is cast as an integral extension of the vane extension. However, difficulties in cooling, manufacturing and attaching the nozzle stage segments to the turbine casings occur with that configuration. For example, the vane extension in the outer band has apertures for flowing the cooling medium into the leading edge cavity of the vane. These cooling apertures cause stress because the load support path for the vane and inner band portions of the nozzle stage segment pass through the hot leading edge and fillet. The cooling flow apertures through each vane extension also afford an undesirable pressure loss as the cooling steam flows from the outer band into the vane. Moreover, from a review of U.S. Pat. No. 5,634,766, it will be appreciated that the location of the forward support hook renders insertion of the impingement cooling insert into the leading edge cavity difficult. Further, the integral mounting of the forward hook on the vane complicates the manufacture and assembly of the nozzle stage segment, affording unnecessary complexity and a substantial number of parts necessary to work around the hook that is cast integrally on the nozzle vane extension.

BRIEF SUMMARY OF THE INVENTION

In accordance with a preferred embodiment of the present invention, the mechanical attachment of the nozzle stage segment to the outer fixed casing of the turbine is accomplished by forward and aft hooks on the outer band, with the forward hook being formed integrally with the cover and the aft hook formed integrally with the outer wall. The vane also includes a vane extension between the wall and cover of the outer band to which the cover with the integral hook is secured, e.g., by welding. The vane extension, however, is spaced back from the leading edge of the vane and the leading edge cavity through the vane. In this manner, the load path extends from the hook through the cover to the vane extension whereby stresses on the hot leading edge and fillet are avoided. That is, the load path includes the first rib between opposite side walls and the first and second cavities of the vane for carrying the load of the cantilevered nozzle. The cover and outer wall are secured, preferably by welding, to one another to define the outer chamber forming part of the closed loop cooling circuit. By locating the forward hook on the outer cover, the impingement insert in the first cavity of the vane can be applied directly. Also, the vane extension does not require apertures for flowing cooling steam into the vane cavities which otherwise would stress the load bearing leading edge of the vane. Also, the number and complexity of the parts is significantly reduced. For example, a single impingement plate can be formed and provided in the outer band chamber about the vane extension. Further, the segment casting is greatly simplified.

In a preferred embodiment according to the present invention, there is provided a nozzle stage segment for a gas turbine, comprising inner and outer bands spaced generally radially from one another and a nozzle vane extending between the bands, the nozzle vane having leading and trailing edges, the outer band including a wall for defining a portion of a hot gas flow path through the turbine and an outer cover radially outwardly of the wall defining a chamber with the wall for forming part of a closed loop cooling circuit through the nozzle stage segment, the outer cover having a generally axially forwardly directed hook for structurally attaching the nozzle stage segment to a support on the turbine.

In a further preferred embodiment according to the present invention, there is provided a nozzle stage segment for a gas turbine, comprising inner and outer bands spaced generally radially from one another and a nozzle vane extending between the bands, the nozzle vane having leading and trailing edges, the outer band including a wall, a vane extension extending generally radially outwardly of the wall, and an outer cover radially outwardly of the wall, the outer cover having a generally axially forwardly directed hook for attaching the nozzle stage segment to a support on the turbine, the vane extension and the outer cover being secured to one another to define a structural load bearing path through the outer cover between the hook and the vane.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary side elevational view of a nozzle stage segment constructed in accordance with the present invention;

FIG. 2 is an exploded perspective view of various elements forming the nozzle stage segment illustrated in FIG. 1;

FIG. 3 is a perspective view of an outer cover for the outer band of the nozzle stage segment; and

FIG. 4 is a perspective view illustrating the cover secured to the segment.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated a nozzle stage segment, generally designated 10, comprised of an outer band 12, an inner band 14 and a nozzle vane 16 extending generally radially between the outer band 12 and inner band 14. It will be appreciated that the nozzle stage segment illustrated in FIG. 1 is one of an annular array of segments arranged about a rotor axis and about a rotor, portions of which are illustrated at 18. As conventional, the rotor includes a plurality of buckets, one being partially illustrated at 20, for rotation about the turbine axis, the buckets 20 and vanes 16 lying in a hot gas path 22. The direction of flow of the hot gas is indicated by the arrow 24.

The nozzle stage segments 10 are secured to a fixed casing of the turbine surrounding the nozzle stages and buckets. Particularly, the fixed casing includes forward and aft recesses or grooves 26 and 28, respectively, for receiving forward and aft hooks 30 and 32 by which each nozzle segment is supported from the fixed casing. The forward and aft hooks form part of the outer band and it will be appreciated that the vane 16, inner band 14 and diaphragm 34 are cantilevered from the forward and aft hooks of the fixed casing.

Referring now to FIG. 2, the outer band 12 comprises an outer wall 36 and an outer cover 38 defining in assembly a chamber therebetween. The inner band 14 is formed of an inner wall 42 and an inner cover 44 defining a chamber therebetween. From a review of FIG. 2, it will be appreciated that the vane 16 and outer and inner walls 36 and 42, respectively, comprise an integral casting. Additionally, the vane 16 is divided into a plurality of cavities, including a leading edge cavity 48, intermediate cavities 50, one or more aft cavities 64 and a trailing edge cavity 54. The cavities are separated one from the other by radially extending ribs extending between opposite side walls of the vane 16. A vane extension 56 is also illustrated in FIG. 2 and is defined by the first rib 58 extending through the vane from the leading edge 60 thereof. The vane extension 56 includes opposite side walls contoured in the shape of the vane 16 and having an intermediate rib 61 and aft rib 62. The aft cavities 64 open to the chamber between the outer wall 36 and cover 38. The trailing edge cavity 54 extends along the trailing edge of the vane 16 and forms a separate vane extension 55 in the region of the chamber between wall 36 and cover 38.

The outer cover 38 is preferably comprised of an integral casting including the forward hook 30 and an extension 66 having a corresponding shape as vane extension 56 to receive the upper end of vane extension 56. Cover 38 also includes a cooling medium inlet, for example, steam inlet 68 and a separate steam exit cover 70 having a steam exit 72. The steam exit cover 70, in final assembly, overlies the extension 66. An impingement plate 73 lies in the chamber between the wall 36 and cover 38 and is of a single unitary one-piece construction having a central opening for surrounding the extension 56. Standoffs or pins 74 are provided to support the impingement plate 73 in spaced relation to the wall 36, it being appreciated that the impingement plate has a plurality of apertures or openings therethrough for flowing steam from between the cover 38 and the impingement through the apertures for impingement cooling wall 36.

The cavities through the vane 16 with the exception of the trailing edge cavity open into the chamber between the inner wall 42 and inner cover 44, respectively. The leading edge and aft cavities 48 and 64 conduct the cooling steam through the vane and inserts in the vane, not shown, for impingement cooling of the side walls of the vane 16. The steam flows from the cavities through steam guides, not shown, into the inner chamber on the radially inner side of the impingement plate 75. The steam then flows through the apertures of the impingement plate 75 for impingement cooling of the inner wall 42 and is returned through the vane via the intermediate steam return cavities 50 which empty the steam from the vane through the steam outlet 72.

It will be appreciated that in accordance in the present invention, the forward hook 30 forms an integral part of the cover casting, while the aft hook 32 forms an integral part of the nozzle stage segment casting and particularly of the outer wall 36. As illustrated in FIG. 4, the vane extension 56 is received within the opening of the extension 66. Preferably, the cover is welded to the wall 36 about the adjoining margins along the forward and aft edges, as well as along the lateral slash faces. Additionally, and importantly, the side walls of the vane extension 56 are welded, for example, by E-beam welding, to the wall surfaces of the extensions 66. By welding the extensions to one another, it will be appreciated that the load bearing path from the forward hook 30 extends through the welded extensions directly to the first rib 58 of the vane 16. Moreover, by forming the forward hook 30 on the cover 38 rather than on the integrally cast vane segment as in the prior U.S. Pat. No. 5,634,766, the load bearing path is not interrupted by apertures necessary to provide a path for the cooling medium for flow into the vane. As illustrated in FIG. 2, the cooling steam passes through the openings of the impingement plate 73 for impingement cooling of the outer wall 36 and then flows through the cavities 48 and 64 for flow generally radially inwardly through the vane 16. The need for apertures in the vane extension which would otherwise interrupt the load bearing path is entirely eliminated.

While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. 

What is claimed is:
 1. A nozzle stage segment for a gas turbine, comprising: inner and outer bands spaced generally radially from one another and a nozzle vane extending between said bands, said nozzle vane having leading and trailing edges, said inner band and said nozzle vane being cantilevered radially inwardly from said outer band; said outer band including a wall for defining a portion of a hot gas flow path through the turbine and an outer cover radially outwardly of said wall defining a chamber with said wall for forming part of a closed loop cooling circuit through said nozzle stage segment, said cover having an inlet for flowing a cooling medium into said chamber; said outer cover having a generally axially forwardly directed hook for structurally attaching the nozzle stage segment to a support on the turbine.
 2. A nozzle stage segment according to claim 1 wherein said inner band includes an inner wall for defining another portion of the gas flow path through the turbine and an inner cover radially inwardly of said inner wall for defining an inner chamber with said inner wall, said vane having opposite side walls spaced from one another defining at least one cavity therethrough in communication with said outer chamber for supplying a cooling medium from said outer chamber through said one cavity to said inner chamber and a second cavity therethrough in communication with said inner chamber for returning the cooling medium through said vane to a cooling medium outlet port in said outer cover.
 3. A nozzle stage segment according to claim 1 including an axially aft directed hook carried by said outer wall for attaching the nozzle stage segment to another support on the turbine.
 4. A nozzle stage segment according to claim 1 wherein said outer cover and said outer wall are welded to one another, said hook being integrally cast with said outer cover.
 5. A nozzle stage segment according to claim 1 wherein said vane has a vane extension between said outer cover and said wall, said outer cover and said vane extension being welded to one another.
 6. A nozzle stage segment according to claim 5 wherein said outer cover has an opening for receiving said vane extension, margins of said outer cover opening and said vane extension being welded to one another.
 7. A nozzle stage segment for a gas turbine, comprising: inner and outer bands spaced generally radially from one another and a nozzle vane extending between said bands, said nozzle vane having leading and trailing edges; said outer band including a wall, a vane extension extending generally radially outwardly of said wall, and an outer cover radially outwardly of said wall defining a chamber with said wall, said cover having an inlet for flowing a cooling medium into said chamber; said outer cover having a generally axially forwardly directed hook for attaching the nozzle stage segment to a support on the turbine, said vane extension and said outer cover being secured to one another to define a structural load bearing path through said outer cover between said hook and said vane.
 8. A nozzle stage segment according to claim 7 wherein said vane extension and said cover are welded to one another.
 9. A nozzle stage segment according to claim 7 wherein said outer cover has an opening for receiving said vane extension, said outer cover and said vane extension being welded to one another about said opening.
 10. A nozzle stage segment for a gas turbine, comprising: inner and outer bands spaced generally radially from one another and a nozzle vane extending between said bands, said nozzle vane having leading and trailing edges; said outer band including a wall for defining a portion of a hot gas flow path through the turbine and an outer cover radially outwardly of said wall defining a chamber with said wall for forming part of a closed loop cooling circuit through said nozzle stage segment; said outer cover having a generally axially forwardly directed hook for structurally attaching the nozzle stage segment to a support on the turbine; said vane having spaced opposite side walls and a plurality of ribs defining a plurality of discrete, generally radially extending cavities, one of said cavities extending between said side walls along the leading edge of said vane and forwardly of a first rib of said plurality of ribs thereof to define a leading edge cavity, said vane having a vane extension between said outer wall and said cover opening through said outer cover, said leading edge cavity opening into said chamber through said outer wall forwardly of said vane extension.
 11. A nozzle stage segment according to claim 10 wherein said side walls adjacent the trailing edge of said vane and aft of an aft rib of said plurality of ribs define a trailing edge cavity of said plurality of said cavities, a second vane extension between said outer wall and said cover opening through said cover and defining a continuation of said trailing edge cavity, and at least another of said cavities opening through said outer wall into said chamber between the vane extensions.
 12. A nozzle stage segment for a gas turbine, comprising: inner and outer bands spaced generally radially from one another and a nozzle vane extending between said bands, said nozzle vane having leading and trailing edges; said outer band including a wall for defining a portion of a hot gas flow path through the turbine and an outer cover radially outwardly of said wall defining a chamber with said wall for forming part of a closed loop cooling circuit through said nozzle stage segment; said outer cover having a generally axially forwardly directed hook for structurally attaching the nozzle stage segment to a support on the turbine; said vane including a vane extension between said outer wall and said outer cover, said vane extension being welded to said outer cover, said vane having a load-bearing rib extending between opposite side walls thereof and spaced from the leading edge of said vane and in part defining a load-bearing path through said outer cover and said hook for supporting said vane and said inner band from the turbine support.
 13. A nozzle stage segment according to claim 12 wherein said vane has spaced opposite side walls and a plurality of ribs defining a plurality of discrete, generally radially extending cavities, one of said cavities extending between said side walls along the leading edge of said vane and forwardly of said load bearing rib to define a leading edge cavity, said leading edge cavity opening into said chamber through said outer wall forwardly of said vane extension.
 14. A nozzle stage segment according to claim 13 wherein said side walls adjacent the trailing edge of said vane and aft of an aft rib of said plurality of ribs define a trailing edge cavity of said plurality of said cavities, a second vane extension between said outer wall and said cover opening through said cover and defining a continuation of said aft cavity, and at least another of said cavities opening through said outer wall into said chamber between the vane extensions.
 15. A nozzle stage segment for a gas turbine, comprising: inner and outer bands spaced generally radially from one another and a nozzle vane extending between said bands, said nozzle vane having leading and trailing edges; said outer band including a wall for defining a portion of a hot gas flow path through the turbine and an outer cover radially outwardly of said wall defining a chamber with said wall for forming part of a closed loop cooling circuit through said nozzle stage segment; said outer cover having a generally axially forwardly directed hook for structurally attaching the nozzle stage segment to a support on the turbine; and a one-piece impingement plate between said outer cover and said wall.
 16. A nozzle stage segment according to claim 15 wherein said vane includes a vane extension between said outer cover and said wall, said impingement plate having an opening therethrough surrounding said vane extension.
 17. A nozzle stage segment for a gas turbine, comprising: inner and outer bands spaced generally radially from one another and a nozzle vane extending between said bands, said nozzle vane having leading and trailing edges; said outer band including a wall, a vane extension extending generally radially outwardly of said wall, and an outer cover radially outwardly of said wall; said outer cover having a generally axially forwardly directed hook for attaching the nozzle stage segment to a support on the turbine, said vane extension and said outer cover being secured to one another to define a structural load bearing path through said outer cover between said hook and said vane; said vane including a load bearing rib extending between opposite side walls thereof and spaced from a leading edge of said vane, said vane extension having a portion forming an integral extension of said rib and being secured to said cover to define a load bearing path between said hook and said vane. 